The invention relates to a catalytic layer system on a resistive gas sensor, especially on a resistive gas sensor layer based on a titanate, with at least one first layer and a second layer lying over the first layer, where at least one layer exhibits a catalytically active substance.
A gas sensor that is built up on a substrate using thick-film technology is known from EP 360 159 B1. Here, the gas-sensitive layer consisting of SnO.sub.2, ZnO, Fe.sub.2 O.sub.3, TiO.sub.2 or CoO is soaked in a preferably aqueous solution of a platinum compound, such as chloro platinum acid. This is followed by a temperature treatment in the range of -5.degree. C. to 180.degree. C. in a reducing gas atmosphere and at a defined relative humidity. An additional temperature at more than 200.degree. C. is necessary to optimize the desired precipitation of the precious metal in the gas-sensitive layer. Consequently, the entire impregnation process is very cumbersome and is characterized by a large number of individual steps which, based on their abundance alone, are prone to error in production terms. Furthermore, it should be noted that the impregnation process only improves the catalytic activity of the gas-sensitive layer itself in its interaction with the electrodes, but that this arrangement does not provide protection against mechanical influences or against compounds that act as catalyst poisons.
DE 42 40 267 A1 discloses an impregnation technology similar to the aforementioned state of the art. However, it is not used on a gas-sensitive layer, but rather in connection with the catalytic activation of a cermet electrode on an oxygen-ion-conducting solid electrolyte of a gas sensor. In this case, various platinum salt solutions are listed that are applied to a porous cermet electrode. To ensure better penetration into the pores of the electrode, 100 mbar to 1 bar of pressure is applied to the sensor element while it is being soaked in the platinum solution. Again, this is followed by a costly drying and baking procedure, partly in air and partly in a hydrogen stream. The sensor object disclosed in DE 42 40 267 is finger-shaped (a tube that is closed at one end), while the cermet electrodes are generally secured to the closed end of the finger-shaped object. Consequently, soaking of the cermet electrode is preferably achieved by dipping the electrodes, which are arranged on the spherical surface of the sensor body, into the fluid platinum solution. As this can easily lead to droplet formation, it is difficult to ensure a reproducible application quantity and, consequently, a uniform concentration of the catalytically active components penetrating into the electrode layer using this application technology. A porous, inert coating, which serves as a protective layer, is subsequently applied in one or more separate steps to this catalytically activated cermet electrode (e.g., by plasma spraying of a magnesium spinel).
Coating layers made of oxide mixtures of SiO.sub.2, Al.sub.2 O.sub.3 and BaO on electrical conductors and/or semiconductors are known from DE 195 00 235 A1. More or less porous layers are generated, depending on the BaO percentage and the baking temperature. A coating layer according to DE 195 00 235 for use on a gas-sensitive layer, e.g., of strontium titanate, only encompasses a protective function, as the catalytically active component is lacking. It also has the disadvantage that a mechanically tension-free bond with the underlying gas sensor layer cannot be achieved, as this oxide mixture does not contain the ceramic material of the gas-sensitive layer. Consequently, the SiO.sub.2 --Al.sub.2 O.sub.3 --BaO coating layer known from DE 195 00 235 lacks the attribute of catalytic action and a component that provides for favorable adhesion of the layer sequence to a sensitive layer.
DE 38 13 930 C2 describes a well-adhering electrode structure on an oxygen-ionconducting solid electrolyte made of ZrO.sub.2 that is covered by a protective layer consisting of a ceramic material or by a ceramic "spacer layer". In a special embodiment, a catalyst layer that contains a platinum group metal is arranged over this first layer. This catalyst layer is, in turn, coated with a porous protective layer made of sprayed-on spinel (example 3). The entire layer structure consisting of a two-layer cermet electrode, a ceramic spacer layer, a catalyst layer and a ceramic coating layer is extremely complex and, due to the use of various application technologies (printing, plasma spraying), very costly in its manufacture.
Consequently, the objective of the invention is to provide a catalytically active layer system for a resistive gas sensor which, while possessing optimized diffusion properties (porosity), exhibits favorable adhesion to the underlying layer or interface by minimizing the differences in the thermal expansion behavior of the materials, and which exhibits favorable mechanical resistance to the possible effects of abrasive particles over long periods of time.